This invention relates to apparatus, such as contactors, for switching electric current; and more particularly to a control circuit for applying electricity to a coil in the apparatus to open and close a set of switch contacts.
The application of electricity to motors and other large loads often is controlled by type of relay known as a contractor. The contactor has one or more sets of electrical switch contacts which typically are spring biased into an open state. When a solenoid coil of the contactor is energized, an electromagnetic field is produced which forces the switch contacts closed. Thus the contactor enables a relatively small current and voltage applied to the coil to switch a much larger current and/or voltage to the load.
With some types of contactors a greater current is required to initially close the contacts than is required thereafter to maintain the contacts in the closed state. As a consequence in some applications, such as battery powered equipment, it is desirable to reduce the coil current after the contacts close in order to conserve power. One technique for controlling the current is to pulse width modulate the electricity applied to the contactor coil and vary the duration of the pulses to alter the magnitude of the current applied to the coil.
With pulse width modulation, the energy stored in the coil can be employed to produce "flyback" current during the off period of each pulse cycle in order to maintain the electromagnetic field that keeps the contacts closed. Thus a low impedance feedback path is established around the coil for this flyback current. However that low impedance feedback path has the drawback of slowing the decay of the electromagnetic field when the contacts are to open. This slows the physical separation of the contacts and increases arcing conditions between the separating contacts.
In addition, external devices, such as transient suppressors, connected across the coil terminals of conventional contactors also may adversely affect the speed at which the contacts open to turn off the load.